Device-to-device communication (D2D) is a component of existing wireless technologies, including ad hoc and cellular networks. Examples include Bluetooth and several variants of the IEEE 802.11 standards suite such as WiFi Direct. These systems typically operate in unlicensed spectrum. D2D communications may also operate as an underlay to cellular networks as a means to take advantage of the proximity of communicating devices and at the same time to allow devices to operate in a controlled interference environment. Such device-to-device communication may share the same spectrum as the cellular system by, for example, reserving some of the cellular uplink resources for device-to-device purposes. Allocating dedicated spectrum for device-to-device purposes, however, is not a desirable solution as spectrum is a scarce resource. Dynamic spectrum sharing between the device-to-device services and cellular services provides flexibility and higher spectrum efficiency.
D2D communication may offer a variety of services to peer devices. Some non-limiting examples of such services include public safety and disaster relief (also known as national security and public safety (NSPS)), relaying function for coverage extension, proximity based social networking, cooperative positioning, and so on. D2D applications may include direct discovery and direct communication. In both cases, a transmitter sends D2D signals that are directly received at least by the intended receivers. D2D devices may operate in multi-carrier scenarios where cellular and/or D2D is configured to operate on multiple carriers. Such carriers do not necessarily belong to a single operator and are not necessarily coordinated and synchronized. D2D devices may operate on a carrier that is not the carrier of the device's serving cell.
D2D communication may refer to direct communication between devices. D2D devices operate within a radio access network. In cellular network assisted device-to-device communications (also called D2D communications as a cellular underlay), user equipment (UE) in the vicinity of each other can establish a direct radio link (D2D bearer). While UEs communicate over the D2D “direct” bearer, they also maintain a cellular connection with their respective serving base station (eNB).
In this way the cellular radio access network (RAN) can assist and supervise the UEs in allocating time, frequency, and code resources for the D2D bearer. Also, the cellular network may determine whether the D2D pair should use the direct link or communication should take place via the eNB. The network may also set the maximum power level that the D2D pair may use for the D2D bearer. Thus, network assisted D2D communications may take advantage of the short distances between devices and reuse cellular spectrum while at the same time protecting the cellular layer from potentially harmful interference caused by the D2D link.
D2D communication may support two different operational modes. In the first mode, the location of the resources for transmission of the scheduling assignment by the broadcasting UE comes from the eNodeB. The location of the resource(s) for transmission of the D2D data by the broadcasting UE comes from the eNodeB. In the second mode, a resource pool for scheduling assignment is pre-configured and/or semi-statically allocated. The UE selects a resource for scheduling assignment from the resource pool.
D2D communication may operate in multicarrier or carrier aggregation (CA) networks. In multicarrier or carrier aggregation networks, the UE is able to receive and/or transmit data to more than one serving cell (i.e., a CA capable UE may operate with more than one serving cell).
The carrier of each serving cell may be referred to as a component carrier (CC). A component carrier generally refers to an individual carrier in a multi-carrier system. Carrier aggregation may also be referred to as “multi-carrier system”, “multi-cell operation”, “multi-carrier operation”, or “multi-carrier” transmission and/or reception.
Carrier aggregation may transmit signaling and data in both the uplink (UL) and downlink (DL) directions. One of the component carriers is the primary component carrier (PCC) (also referred to as primary carrier or anchor carrier). The remaining carriers are called secondary component carriers (SCC) (also referred to as secondary carriers or supplementary carriers). The serving cell may be interchangeably referred to as primary cell (PCell) or primary serving cell (PSC). Similarly, the secondary serving cell may be interchangeably referred to the as secondary cell (SCell) or secondary serving cell (SSC).
Generally, the primary or anchor component carrier carries the essential UE specific signaling. In carrier aggregation, the primary component carrier (e.g., PCC or PCell) exists in both uplink and downlink directions. In a scenario with a single uplink component carrier, the single uplink is the PCell. The network may assign different primary carriers to different UEs operating in the same sector or cell.
The serving radio network node (e.g., eNodeB in LTE) may use a configuration procedure to configure a carrier aggregation UE with one or more SCells (DL SCell, UL SCell, or both). The eNodeB may use a de-configuration procedure to de-configure or remove one or more already configured SCells (DL SCell, UL SCell, or both). The configuration or de-configuration procedure may also be used to change the current multi-carrier configuration (e.g., for increasing or decreasing the number of SCells or for swapping the existing SCells with new ones).
D2D UEs transmit D2D signals or channels in the uplink part of the spectrum. A D2D UE may operate in a half-duplex mode (i.e., the UE can either transmit D2D signals/channels or receive D2D signals/channels). D2D relay UEs may relay some signals to other D2D UEs. D2D signals may include control information, some of which is transmitted by D2D UEs and some of which is transmitted by eNodeBs (e.g., D2D resource grants for D2D communication may be transmitted via cellular downlink control channels). D2D transmissions may occur on resources which are configured by the network or selected autonomously by the D2D UE.
D2D communication refers to transmitting, by a D2D transmitter, D2D data and D2D control information with scheduling assignments (SAs) to assist D2D receivers of the D2D data. D2D data may be transmitted according to configured patterns and may be transmitted relatively frequently. Scheduling assignments may be transmitted periodically. In some examples of operation, D2D transmitters that are within the network coverage may request eNodeB resources for their D2D communication transmissions and receive in response D2D resource grants for scheduling assignments and D2D data. In other examples of operation, an eNodeB may broadcast D2D resource pools for D2D communication.
D2D discovery messages may be transmitted in relatively infrequent periodic subframes. An eNodeB may broadcast D2D resource pools for D2D discovery, both for reception and transmission.
Power control is a consideration for D2D communication. In LTE, uplink power control is specified. An objective is to control the UE transmit power of different uplink physical channels including PUSCH and PUCCH.
According to third generation partnership project (3GPP) TS 36.213 v12.2.0, the setting of the UE Transmit power for a Physical Uplink Shared Channel (PUSCH) transmission may be defined as follows.
If a UE transmits PUSCH without a simultaneous PUCCH for the serving cell c, then the UE transmit power PPUSCH,c (i) for PUSCH transmission in subframe i for the serving cell c is given by
            P              PUSCH        ,        c              ⁡          (      i      )        =      min    ⁢                  {                                                                              P                                      CMAX                    ,                    c                                                  ⁡                                  (                  i                  )                                                                                                                                                                                            10                        ⁢                                                                              log                            10                                                    ⁡                                                      (                                                                                          M                                                                  PUSCH                                  ,                                  c                                                                                            ⁡                                                              (                                i                                )                                                                                      )                                                                                              +                                                                        P                                                      O_PUSCH                            ,                            c                                                                          ⁢                                                  (                          j                          )                                                                    +                                                                                                                                                                                                                α                            c                                                    ⁡                                                      (                            j                            )                                                                          ·                                                  PL                          c                                                                    +                                                                        Δ                                                      TF                            ,                            c                                                                          ⁡                                                  (                          i                          )                                                                    +                                                                        f                          c                                                ⁡                                                  (                          i                          )                                                                                                                                                        }            ⁡              [        dBm        ]            
If a UE transmits PUSCH simultaneous with PUCCH for the serving cell c, then the UE transmit power PPUSCH,c (i) for the PUSCH transmission in subframe i for the serving cell c is given by
            P              PUSCH        ,        c              ⁡          (      i      )        =      min    ⁢                  {                                                                              10                  ⁢                                                            log                      10                                        ⁡                                          (                                                                                                                                  P                              ^                                                                                      CMAX                              ,                              c                                                                                ⁡                                                      (                            i                            )                                                                          -                                                                                                            P                              ^                                                        PUCCH                                                    ⁡                                                      (                            i                            )                                                                                              )                                                                      ,                                                                                                                                                                            10                        ⁢                                                                              log                            10                                                    ⁡                                                      (                                                                                          M                                                                  PUSCH                                  ,                                  c                                                                                            ⁡                                                              (                                i                                )                                                                                      )                                                                                              +                                                                        P                                                      O_PUSCH                            ,                            c                                                                          ⁢                                                  (                          j                          )                                                                    +                                                                                                                                                                                                                α                            c                                                    ⁡                                                      (                            j                            )                                                                          ·                                                  PL                          c                                                                    +                                                                        Δ                                                      TF                            ,                            c                                                                          ⁡                                                  (                          i                          )                                                                    +                                                                        f                          c                                                ⁡                                                  (                          i                          )                                                                                                                                                        }            ⁡              [        dBm        ]            
To enable uplink power control operation, a UE may be configured with one or more parameters. For example, the UE may derive path loss (PL) in the above expressions based on CRS power and RSRP measurement on a serving cell in which it operates. As a particular example, according to TS 36.213 v12.2.0, PLc is the downlink path loss estimate calculated in the UE for serving cell c in dB and PLc=referenceSignalPower is the higher layer filtered RSRP, where referenceSignalPower is provided by higher layers and RSRP is measured by the UE on the downlink CRS signals of a serving cell for which uplink power control is performed by the UE.
A carrier aggregation UE may independently perform power control in each serving cell (i.e., PCell and SCell(s)) for different control channels. Path loss may be derived based on RSRP and referenceSignalPower of the downlink serving cell (i.e. DL CC) which is linked to an uplink serving cell (i.e. UL CC) for which the power control is performed. The linkage between the downlink and uplink carriers may be signaled to the UE by a higher layer.
A D2D UE configured on an uplink carrier may perform certain measurements on downlink signals on the corresponding downlink carrier (i.e., on DL of the serving cell of the UE). The D2D UE may use these measurements for radio operations at least partly related to D2D operation. For example, such radio operations may include: deriving path loss for power control or for transmitting signals on D2D links with certain power; UE transmit timing adjustment of D2D signals; and estimation of signal strength or signal quality with respect to a network node.
Typically, D2D operation is configured on one of the serving carriers of the UE (e.g., on UL PCell in single carrier or UL SCell in carrier aggregation). On a serving carrier, the UE is configured for at least wireless access network operation (i.e., cellular operation, or non-D2D operation). In this configuration, a UE can perform downlink measurements needed for D2D operation on the downlink of the serving cell.
However, a D2D UE operating in single carrier operation for a wireless access network may also be configured for D2D operation on a carrier which is different from the serving carrier of the UE. Similarly, the D2D UE operating in carrier aggregation mode for a wireless access network may also be configured for D2D operation on a carrier which is different than any of the serving carriers of the UE. In these scenarios, there is no downlink cell or carrier associated with the uplink cell or carrier on which the D2D UE can perform downlink radio measurements.
Thus, the UE behavior in terms of how to perform downlink measurements for D2D operation is unspecified and undefined. Therefore, in such a scenario, the D2D operation either cannot be performed or it may significantly degrade the system performance. For example, because of a lack of path loss measurement with respect to the downlink cell that should be linked to the uplink cell on which D2D operation is configured, the UE may transmit at full output power. This may cause interference in the uplink at the receiver of the radio network nodes (e.g., neighboring base stations).